Conventionally, a starter for starting an internal combustion engine typically comprises a motor, a one-way clutch connected to the motor and driven by the same via a reduction gear unit consisting of a planetary gear device or the like, a pinion gear connected to the one-way clutch, a solenoid for displacing the pinion gear via a shift mechanism in response to an engine start switch signal so that the pinion gear meshes with a ring gear of the engine, and others.
To minimize total size of the starter, a coaxial engine starter in which the aforementioned component parts are arranged coaxially has been proposed for example in Japanese Patent Application Laid Open Publication No.8-319926 filed by the Applicant (Assignee) of this application. In this engine starter, the solenoid is disposed around the starter's output shaft and accommodated in a shift chamber housing which also accommodates the pinion gear, shift mechanism, reduction gear unit and others, while the electric motor is accommodated in a substantially cylindrical motor casing. Between the shift chamber housing and the motor casing is disposed a brush holder stay which can be made of resin material by molding.
A terminal for connection to an outside power supply (typically a battery) is favorably mounted to the brush holder stay. The power supply terminal projects radially outwardly with respect to the motor casing and the brush holder stay is formed with a corresponding radially protruding portion to support the terminal. A contact unit for selectively supplying electric power to the motor is also favorably mounted to the brush holder stay near the power supply terminal. The brush holder stay may be formed with a cavity for mounting the contact unit therein, with the cavity opened toward the motor casing for the reasons such as convenience in mounting the contact unit therein and/or easiness in molding the brush holder stay. In the assembled state of the starter the cavity must be covered so as to prevent water and/or dust from entering it, but the cavity typically extends to the protruding portion and cannot be covered by the motor casing. Therefore, a rubber cover, for example, is used to close the cavity in such a manner that the cover is pushed in the axial direction against the brush holder stay by the motor casing in order to achieve watertight engagement between the cover and the brush holder stay.
However, the axial force applied to the cover by the motor casing may not work effectively to press the peripheral part of the cover against the brush holder stay, and therefore it has been difficult to reliably achieve watertight contact between the brush holder stay and the cover for covering the cavity of the brush holder stay.